CMP is a combination of chemical reaction and mechanical buffing. A conventional CMP system includes a polishing head with a retaining ring that holds and rotates a substrate (also referred to interchangeably as a wafer) against a pad surface rotating in the opposite direction or same direction. The pad can be made of cast and sliced polyurethane (or other polymers) with a filler or a urethane coated felt.
During rotation of the substrate against the pad, a slurry of silica (and/or other abrasives) suspended in a mild etchant, such as potassium or ammonium hydroxide, is dispensed onto the pad. The combination of chemical reaction from the slurry and mechanical buffing from the pad removes vertical inconsistencies on the surface of the substrate, thereby forming an extremely flat surface.
However, conventional CMP systems have several shortcomings including process instability that can lead to inconsistent polish profiles of substrates; table-to-table and tool-to-tool variation that can lead to inconsistent polish profiles of substrates processed on different CMP systems; and process optimization difficulties that make it difficult to balance pressure within air-pressurized chambers due to a plurality of pressure controllers.
FIG. lA is a block diagram illustrating a cross section of a prior art polishing head 100 that exhibits the above-mentioned deficiencies. A retaining ring 125 is cylindrical in shape and holds a substrate 120 (also referred to as a wafer) in place during CMP. An air pressure/force balancing method, as indicated by the arrows in FIG. lA, is used to maintain a downward pressing force against a shaft and the substrate 120 during CMP. In addition, to prevent a plate 140 from ballooning out of the polishing head 100, supplied pressure exerts an upward force.
However, these above-mentioned forces are subject to process instability, which can lead to inconsistent polish profiles of substrates. Specifically, the above-mentioned forces are each powered by air pressure administered by air pressure controllers. The controllers each have their own tolerances that can lead to errors in the amount of air pressure applied. For example, if the pressure in region 105 is greater than the pressure in region 115, the plate 140 is placed in a position that is lower than expected. A rubber insert 130 is formed as shown in FIG. 1B (and is different from FIG. 1C when the plate 140 is placed in the expected position). In the condition shown in FIG. 1B, the plate 140 compresses the edge of rubber insert 130 due to the pressure difference between region 105 and 115. This compressing force gives a pressure on the edge of the substrate 120 that is different from a pressure on the other region provided by air pressure in region 115. As a result, excess pressure is applied on an edge of the substrate 120 and it increases a polishing rate of the substrate 120.
Further, there can be additional variation between conventional CMP systems that lead to inconsistent profiles between substrates. In addition, it can be hard to optimize the process in conventional CMP systems so that the forces required are adequately and consistently balanced.
Another shortcoming of conventional CMP systems is that CMP heads always get lowered to the same position even though the pads wear down over time. This can lead to the insufficient polishing of substrates.
Therefore, a system and method are needed that overcome the above-mentioned deficiencies.